Retailers and consumer product manufacturers are actively pursuing technologies based on radio frequency identification (RFID) that help them track the location of products in a supply chain, in warehouses, or on a shop floor. Conventional RFID tags are simple, small passive devices intended as an “electronic barcode” for use in supply chain management. Tags consist of small integrated circuit chips typically attached to small antennae, each capable of transmitting a unique serial number to a reading device in response to a query. Most RFID tags are batteryless, obtaining the power necessary for operation from an external modulated magnetic field, and can often be read at a distance of several meters.
The tags thus serve as means for remotely identifying a particular person or object to which they are attached. Manufacturers and shop owners prefer them to conventional optically-scanned barcodes because they uniquely identify individual items, rather than just product types, and because they can be read indirectly and in high volume. RFID tags are detected with handheld readers that in many cases are bulky and fairly expensive, on the order of $200–500 each. As technology advances, the size and price of readers declines, and eventually readers will be small enough to be incorporated into an item that is less conspicuous and more convenient to carry and operate. RFID tags are already quite common and offer many interesting possibilities that might be of value to consumers. For example, contactless smartcards are like ordinary credit or debit cards but incorporate an RFID tag, so that transactions can be made without requiring physical contact with a reader as with conventional magnetic stripes.
Unfortunately, this technology trend may lead to a serious loss of consumer privacy. A tracking device embedded in a product purchased by the consumer and not removed or deactivated at the store may be used for malicious purposes. In effect, the person carrying or wearing the product can be tracked wherever he or she goes—a privacy invasion of Orwellian proportions.
Several different approaches to the RFID privacy problem have been pursued in the past. U.S. Pat. No. 6,121,544 to Petsinger teaches a shielding device that effectively prevents communication with contactless smartcards or RFID tags. The shield is electrically conductive and has a high magnetic permeability so that the electromagnetic fluctuations that normally power the smartcard or tag are blocked. Similarly, any signal emitted by the tag or smartcard is also isolated from the outside world. To shield a tag, though, a user must know where the tag is located and then deliberately insert the tag into the shielding device. This has to be done for all tags a person is carrying in their clothing or on other objects.
A different strategy is proposed in U.S. Patent Application 2002/0100359 by Reade et al. and in the article by A. Juels, R. L. Rivest, and M. Szydlo: “The Blocker Tag: Selective Blocking of RFID Tags for Consumer Privacy” in the 8th ACM Conference on Computer and Communications Security, p. 103–111, ACM Press, 2003. These references teach a type of RFID tag that essentially mounts a denial of service attack on a RFID reader so the reader cannot capture the unique code that it would usually be able to capture from individual RFID tags. By flooding the reader with responses instead of transmitting one unique code, the blocker tag simulates the presence of a very large number of possible tags. While the reader cannot therefore uniquely identify the RFID tag, the blocking tag is basically a jamming (or spamming) device that makes its presence quite clearly known, so the reader is aware that someone wants to prevent their tags from being read. Reade et al. describe a variety of structures containing the jamming devices, each designed to resemble an item typically worn or carried by a consumer, including a cell phone, pager, camera, wristwatch, bracelet, belt, pen, and so forth.
A third approach is to employ cryptographic methods to allow tags to interact in a way that protects privacy better while providing the desired active functionality. U.S. Patent Application 2004/0054900 by He describes a complex system that employs public-private key encryption to exchange challenge/response message exchanges between RFID tags on manufactured merchandise and merchants' RFID interrogators. An article by D. Henrici and P. Muller, “Hash-Based Enhancement of Location Privacy for Radio-Frequency Identification Devices using Varying Identifiers” in the Proceedings of the Second IEEE Annual Conference on Pervasive Computing and Communications Workshops (PERCOMW'04), 2000, teaches a scheme for providing location privacy as well as data privacy. The general idea of the Henrici reference is to change the ID of a tag on every read attempt in a secure manner. Both these references require RFID tags that are more complex than those currently in use. Further, Henrici relies on read-write RFID tags and assumes that identical hash functions are used in all tags and readers; it does not solve the problem of RFID privacy when tags are read-only (as are the vast majority of today's tags) or when different types of tags are used. An item may contain multiple tags, e.g. from the manufacturer, transportation company, or retailer, and it is unlikely that they all have identical hash functions or that they are manufactured by the same RFID tag maker.
RFID chips are getting so small and so inexpensive that they can even be embedded in paper, e.g. printed tickets and ordinary paper currency. Hitachi manufactures an RFID chip 0.4 mm square that stores a unique ID number capable of individually identifying trillions of trillions of objects with no duplication. Financial privacy concerns are among the most seriously held consumer opinions, and often determine whether and how fast a potentially invasive technology is adopted.
An article by A. Juels and R. Pappu, “Squealing Euros: Privacy Protection in RFID-Enabled Banknotes”, in the 7th International Conference on Financial Cryptography, 2003, p. 103–121, emphasizes this concern. Juels et al. describe a cryptographic system for hiding the identity of high-denomination Euro banknotes that include embedded RFID tags. In this proposed scheme, the banknote's serial number is transmitted in encrypted form but is re-encrypted on request. Since the encrypted value changes and the encryptions cannot be readily inverted, there is no way to determine if two encrypted values were transmitted by the same banknote. However, this scheme requires cooperation from tag manufacturers (so that all tags have the required computational capabilities), as well as reader manufacturers and the law enforcement agencies who manage the private keys used in encryption.
These prior art efforts to solve RFID privacy problems are poorly adapted to the basic objective: simply giving the consumer the freedom to decide whether and how to participate in the RFID universe without interfering with, overly complicating, or constraining the design of future RFID systems. The consumer faces the prospect of having tracking devices embedded in everyday items yet not knowing if a given item in fact has an active tracking device. This situation calls for a counter-measure that empowers users to make informed judgments about their privacy.